DDoug - Do you mean run it up in the air without the driveshaft attached to the geabox? What would that show? There wouldnt be much/any load on the tailshaft of the box?
There is some rear differential play. However, I have swapped the diff for a different center and the noises still sound exactly the same. This is a video of the car with the handbrake on with wheels on the ground
http://www.youtube.com/watch?v=aqnbEiv1y5M
There didnt seem to be excessive backlash in the crown & pinion when I had it out.
Linesligner - Thanks for the PM. I am about 30 days out of my 90 day warranty, partly due to the fact that when the box was shipped (to Australia) it was shipped without a x member which lead to me having to fab it up after work
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Clois Harlan - Its a solid axle setup in a 68 mustang, rather than a cobra. The mounting bracket appears to be ok and the box wasnt really moving when it was making all that noise
strictlypersonl - I'll try that today.